Fungi isolated from black walnut branches in Indiana and Tennessee urban areas

Abstract

Thousand Cankers Disease (TCD), caused by interaction of the walnut twig beetle (WTB) and the fungus Geosmithia morbida, threatens the health of high-value eastern black walnut. However, other fungi and insects may also cause dieback and death of the species. Baseline data on fungi and insects colonizing the bark was obtained from branches of black walnut in urban areas with and without TCD (Tennessee and Indiana, respectively) in 2012. Visually healthy trees were chosen for study, with the exception of three TCD symptomatic trees in Tennessee. Trees were treated in June 2012. Treatment consisted of girdling the bark (G) from one of the two branches on a tree chosen for study. The second branch on the tree was not girdled (NG). Indiana trees included fourteen treated trees and three untreated trees. Tennessee trees included nine treated trees and three TCD symptomatic untreated trees. The study branches were cut from trees in September 2012. A drawknife was used to expose damage on inner bark of two branch segments from each of two branches from each study tree. Seven types of bark damage (including two insect-related) were characterized for Indiana branch segments, while seven other types (including two insect-related) were described for Tennessee branch segments. Buprestid-like damage was the only damage type common to both states. For Indiana, damage frequencies were similar for G and NG branches but lower for untreated trees. Typical G. morbida cankers were found on branches of both visually healthy and TCD trees in Tennessee. Fungal isolations were attempted from representatives of all damage types from each state. Indiana isolates included 24 Ascomycete species from 16 genera. Tennessee isolates included 64 Ascomycete species from 28 genera. In Tennessee, G. morbida was isolated from three non-insect damage types and from > 50% of WTB damage on G branches. Known walnut pathogens Fusarium solani and Botryosphaeria dothidea were isolated from several damage types on branch segments from both Indiana and Tennessee. The results of this study indicate a need for further research into the role of these fungi in branch dieback, particularly when P. juglandis and G. morbida are present, and their relationship to development of TCD-like symptoms on black walnut in its native range.